Efforts to improve on the performance of natural mineral oil-based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for at least fifty years. These efforts have led to the relatively recent market introduction of a number of synthetic lubricants.
In terms of lubricant property improvement, the thrust of industrial research efforts involving synthetic lubricants has been towards fluids exhibiting useful viscosities over a wide temperature range, i.e., improved viscosity index (VI), while also showing lubricities, thermal stabilities, oxidative stabilities and pour points equal to or better than those for mineral oil.
The viscosity-temperature relationship of a lubricating oil is one of the main criteria considered when selecting a lubricant for a particular application. The mineral oils, commonly used as a base for single and multi-grade lubricants, exhibit a relatively large change in viscosity with a change in temperature. Fluids exhibiting such a relatively large change in viscosity with temperature are said to have a low viscosity index (VI). VI is an empirical number which indicates the rate of change in the viscosity of an oil within a given temperature range. A high VI oil, for example, will thin out at elevated temperatures more slowly than a low VI oil. Usually, a high VI oil is more desirable because it has relatively higher viscosity at higher temperature, which translates into better lubrication and better protection of the contacting machine elements, preferably at high temperatures and or at temperatures over a wide range. VI is calculated according to ASTM method D 2270.
PAOs comprise a class of hydrocarbons manufactured by the catalytic oligomerization (polymerization to low-molecular-weight products) of linear α-olefin (LAO) monomers. These typically range from 1-octene to 1-dodecene, with 1-decene being a preferred material, although oligomeric copolymers of lower olefins such as ethylene and propylene may also be used, including copolymers of ethylene with higher olefins as described in U.S. Pat. No. 4,956,122 and the patents referred to therein.
PAO products have achieved importance in the lubricating oil market. Typically there are two classes of synthetic hydrocarbon fluids (SHF) produced from linear alpha-olefins, the two classes of SHF being denoted as PAO and HVI-PAO (high viscosity index PAO's). PAO's of different viscosity grades are typically produced using promoted BF3 or AlCl3 catalysts.
Specifically, PAOs may be produced by the polymerization of olefin feed in the presence of a catalyst such as AlCl3, BF3, or promoted AlCl3, BF3. These catalysts show reactivity toward branched olefins but exhibit higher reactivity toward alpha-olefins. When oligomerizing a feed of linear alpha-olefins with these catalysts, a process-generated side stream of unreacted monomers is produced. Recycling these unreacted monomers is considered disadvantageous because they contain branched or internal olefins which typically are not desired in the production of conventional PAOs since they have adverse effect on final PAO product properties.
Processes for the production of PAOs using metallocene catalysts in the oligomerization of various alpha olefin feeds has been previously disclosed such as in WO 2007-011832, WO 2007-011459, WO 2007-011973, and WO 2008-010862 all of which provide additional background explicitly or through citation of references, for the present invention. Ideally, it is desirable to convert all the alpha-olefin feeds into lube products. However, sometimes, in order to optimize reactor efficiency and reactor capacity, it is desirable to keep the reaction at partial olefin conversion, less than 100% alpha-olefin conversion. Typically the amount of alpha-olefin monomer converted into lubricant-range (C30-C60) polyalphaolefins is less than 80 mol %.
One of the most pressing problems in the industry is availability and cost of feedstock alpha-olefins. The availability of the feed alpha-olefins has been a challenge for the past several years. Although 1-decene is the most desirable feed, and there have been many efforts to mimic the excellent properties of 1-decene oligomers by varying or supplementing the feedstock with other alpha-olefin monomers. The main problem in using alternative feedstocks, e.g., feedstocks based on one or more of C3-C18 alphaolfins, has been to achieve the same properties in the final PAO as achieved by a pure 1-decene feedstock. See, for instance, Published Application Nos. US2007-0225533, US2007-0225534, US2007-0225535 However, even these alternative feedstocks have become scarce. Thus, improved utilization of all feedstocks in PAO oligomerization processes is an area of continued active research.
Additionally, performance requirements of lubricants are becoming increasingly stringent. New PAOs with improved properties, such as high viscosity index (VI), low pour point, reduced volatility, high shear stability, narrow molecular weight distribution, improved wear performance, increased thermal stability, oxidative stability, and/or wider viscosity range, are needed to meet new performance requirements for lubricants. New methods to provide such new PAOs with improved properties are also needed.
Prior specific efforts to prepare various PAOs using particular metallocene catalyst systems include U.S. Pat. No. 6,706,828, where PAOs are produced from meso-forms of certain metallocene catalysts, such as rac-dimethylsilylbis(2-methyl-indenyl)zirconium dichloride in combination with methylalumoxane (MAO) at 100° C. in the presence of hydrogen to produce polydecene; WO 02/14384, which discloses, among other things, in examples J and K the use of rac-ethyl-bis(indenyl)zirconium dichloride or rac-dimethylsilyl-bis(2-methyl-indenyl)zirconium dichloride in combination with MAO at 40° C. (at 200 psi hydrogen or 1 mole of hydrogen) to produce isotactic polydecene reportedly having a Tg of −73.8° C., a KV100 of 702 cSt, and a VI of 296; or to produce polydecene reportedly having a Tg of −66° C., a KV100 of 1624, and a VI of 341, respectively; and WO 99/67347, which discloses, for example, in Example 1 the use of ethylidene bis(tetrahydroindenyl)zirconium dichloride in combination with MAO at 50° C. to produce a polydecene reportedly having an Mn of 11,400 and 94% vinylidene double bond content.
PAOs have also been made using metallocene catalysts not typically known to produce polymers or oligomers with any specific tacticity. Examples include WO 96/23751, EP 0 613 873, U.S. Pat. No. 5,688,887, U.S. Pat. No. 6,043,401, WO 03/020856 (equivalent to US 2003/0055184), U.S. Pat. No. 5,087,788, U.S. Pat. No. 6,414,090, U.S. Pat. No. 6,414,091, U.S. Pat. No. 4,704,491, U.S. Pat. No. 6,133,209, and U.S. Pat. No. 6,713,438.
Additionally, U.S. Pat. Nos. 6,548,723 and 6,548,724 disclose production of oligomer oils using certain metallocene catalysts, typically in combination with methyl alumoxane. In column 20, lines 40 to 44 of U.S. Pat. No. 6,548,724, Examples 10-11 indicate that di-, tri-, or tetra-substitutions on the cyclopentadienyl rings of the metallocenes are useful for production of high viscosity index polyalphaolefins, (in the range of 20 to 5000 cSt at 100° C.) with improved yields whereas penta-alkyl-substituted cyclopentadienyl rings are poor.” Further examples 12 and 13 show production of polydecenes in the absence of hydrogen with reported KV100's of 154 and 114.6. Additionally Examples 14—discloses polymerization of decene with Cp2ZrMe2 or (iPr-Cp)2ZrCl2 with N,N-dimethylanalinium tetra(phenyl)borate at 100° C. or 110° C. to produce polydecenes with reported KV100's of from 5.3 to 11.4 cSt.
In other examples, PCT/US06/21231 and WO2007011459 A1 describes the production of liquids from monomers having 5 to 24 carbon atoms using metallocenes and non-coordinating anion activators, and WO2007011973 A1 describes the production of low viscosity liquids from alpha-olefins using metallocenes.
In many of the process of PAO's made with metallocenes, it is important to fully utilize the alpha-olefins feeds to obtain the optimized process economics.
In particular, what is needed is a process generally applicable to various metallocene catalyst with high efficiency using a diverse monomer feedstock to consistently produce lube base stocks of highest quality.
The present inventors have surprisingly discovered that under appropriate process conditions unreacted monomers generated during the oligomerization of alpha-olefins could be recycled back into the process without any adverse effect on the properties of final product by optionally maintaining a partial purge of these recycled monomers. Thus improved utilization of olefin feed will be achieved and, even more surprisingly, certain important characteristics of the PAO product, such as at least one of Molecular Weight Distribution, Noack Volatility, and Shear Stability, are either equivalent to or even improved from the same process using fresh feed. This is an extremely important result given the current shortage of traditional feedstocks.